Wastewater has been treated with a variety of physical, chemical, and biological means over the last 100 plus years. Wastewater treatment typically includes a preliminary treatment process that screens debris and settles grit, a primary treatment process that physically removes larger particulates, a secondary treatment process that biologically removes smaller organic particulates and dissolved organics and recovers biological growth, optionally a tertiary treatment process that “polishes” secondary effluent by capturing remaining solids and removing remaining nutrients, and a disinfection process prior to discharge to receiving waters. The most common combination of secondary processes today are suspended growth biological treatment systems for oxidizing dissolved and particulate organics, which consist of bioreactors for biological growth and gravity settling clarifiers for recovering biomass. These systems are energy intensive because blowers are required to supply oxygen for oxidation of organic matter.
Suspended growth biological treatment (activated sludge) is conventionally performed in large bioreactors (tanks) that are supplied large amounts of oxygen and are usually open to the atmosphere. Of the 16,600 publically owned wastewater treatment facilities in the US, there are approximately 6,200 activated sludge facilities, ranging from very small (0.001 mgd) to very large (>800 mgd), treating nearly 80% of all flow. Activated sludge consists of microbial communities (primarily heterotrophic bacteria, but also protists, zooplankton, and annelids, and sometimes autotrophic nitrifying bacteria) and inert and biodegradable organic solids as fluffy solids collectively called “flocs” or “mixed liquor suspended solids” (MLSS). The microbial community, under high-rate operating conditions, is responsible for oxidation of both particulate and soluble carbonaceous matter in wastewater. The flocs are retained and separated from the clean effluent, typically via gravity settling. Excess growth of organisms and accumulation of solids are regularly removed from the system as excess activated sludge. Wastewater treatment in this manner is an energy intensive process, as up to 60% of the energy used at wastewater treatment plants is used to provide oxygen for biological treatment. Other secondary treatment processes include membrane bioreactors, sequencing batch reactors, integrated fixed-film activated sludge, trickling filters, oxidation ditches, treatment lagoons, etc.
Excess activated sludge biomass (waste activated sludge, or WAS) can be digested via anaerobic digestion along with settled solids collected from primary treatment processes (primary solids) to produce biogas that consists mostly of methane. The methane is burned for combined heat and power (CHP) systems at domestic wastewater treatment plants in the US. The theoretically smallest capacity wastewater treatment facility that can benefit from a CHP system is 1 mgd, which would make use of the smallest microturbine on the market today (30 kW). There are approximately 2,900 wastewater treatment facilities with anaerobic digestion processes in the US, with about 60% of plants over 10 mgd including anaerobic digestion. In the last few decades, the biogas produced from anaerobic digestion has begun to be used for heat for digester pre-heating, or for combined heat and power (CHP) systems that generate both heat and electricity at some of the larger domestic wastewater treatment plants in the US. Historically biogas was simply flared. As of 2004, there were nearly 250 digester gas utilization facilities that capitalized on this source of potential energy in some way (heat or CHP), with 76 actual CHP installations nationwide as of 2006 producing a total of 220 MW of power. The number of CHP systems is increasing as a result of today's escalating energy costs, and being implemented at smaller plants. Energy recovery from conventional treatment is typically between 0.15 and 0.2 kWh/m3 of wastewater treated. A facility with anaerobic digestion and CHP can typically offset up to 50% of the energy requirements when compared to a facility without anaerobic digestion and CHP.
Algae (typically microalgae) production for biofuels and/or energy generation is currently in various stages of research and development. Some species of algae possess the ability to synthesize lipid storage compounds under certain growth conditions such as when stressed. High-lipid content algae contains from about 20 to about 50 weight percent lipids. High lipid content algae has a slower growth rate, and is more suitable for use in biofuel production processes than low lipid content algae. While most algae have autotrophic metabolisms (using inorganic carbon and light energy to produce more algae and oxygen), some species have heterotrophic metabolisms (using organic carbon for energy and growth, using oxygen to produce more algae and carbon dioxide), while others have mixotrophic metabolisms (exhibiting both autotrophic and heterotrophic metabolisms either simultaneously or depending on specific conditions).
Most commercial algal production has focused on photosynthetic, autotrophic species such as Botryococcus braunii and is expected to consist of high-rate algal ponds (HRAPs), photobioreactor (PBR) tubes or panels, or some combination of the two. HRAPs are shallow algal ponds or raceways open to the atmosphere, outdoors or in greenhouses, and include simple mechanical mixing. HRAPs typically consist of low concentrations of mixed cultures of algae with a lower lipid content in a much larger footprint, are susceptible to evaporative losses, contamination, competition by undesirable strains of algae, and predation by bacteria and zooplankton. HRAPs are simple and have a lower energy requirement for operation. PBRs are enclosed, transparent tubes, panels or bags tightly configured to maximize solar exposure either outdoors or in greenhouses. PBRs require mixing, flow-through pumping, and sparging of excess oxygen that can be toxic to algal growth. PBRs produce high concentrations of enrichment or pure cultures of algae with a greater lipid content in a much smaller footprint than HRAPs. PBRs also prevent evaporation and minimize contamination, predation and competition. PBRs are complicated and require more energy to operate than HRAPs. Both systems require supplemental carbon and/or nutrient addition to sustain sufficient to optimal algal production.
Recent work has focused on growing heterotrophic algae in fermenter-type bioreactors. The number of candidate strains is far fewer than for phototrophic growth, and include certain species within the genera Chlorella, Tetraselmis, and Nitzschia. Carbon sources can be glucose, glycerol, acetate, some or other carbon source, or waste carbon such as what is in wastewater. Nutrients such as nitrogen and phosphorus are required, and oxygen supply is critical. Without requiring light for growth, these systems are easier to scale and simpler to operate. Culture concentrations can be higher, as light transmission is not a factor, and growth rates can be greater. Much like autotrophic algal species, heterotrophic algal species can be induced to stimulate production of lipids which are valuable for the production of biofuels and other energy commodities. Heterotrophic algae produce carbon dioxide during respiration of organic carbon.
Algae production for biofuel energy has not yet reached large-scale commercialization due to technical and economic challenges. Biofuel from algae remains attractive because algae has dramatically more potential oil yield (between 1,000 and 4,000 gallons/acre/year) than the next highest-yielding biomass feedstock (oil palm, at 635 gallons/acre/year). Technical challenges for commercial algal production are primarily associated with the performance of dewatering technologies. Operating costs for algal production, including supplementation with carbon and nutrients, can also be high. Waste carbon sources (e.g., carbon dioxide from gaseous emissions from power plants for autotrophs; organics in industrial and domestic wastewaters for heterotrophs), have been targeted to reduce operating costs and sequester carbon. Technical challenges for commercial biofuel production from algae are primarily associated with processing/drying dewatered algae and extracting lipids. It has been recognized that direct fermentation/digestion of dewatered algae and electricity generation from the resulting biogas is currently the most cost effective method of recovering energy from algae.
It would be desirable to develop new systems and methods for wastewater treatment that reduce or eliminate the need for oxygen supplementation for removal of organic matter.
It would be desirable to develop new systems or methods for wastewater treatment that reduces nutrients such as nitrogen and phosphorus from effluents discharged to receiving waters.
It would be desirable to develop new systems or methods for wastewater treatment that reduces metals such as chromium, copper, and zinc from effluents discharged to receiving waters.
It would be desirable to develop new systems and methods for wastewater treatment that reduce energy use, produce energy to offset energy needs for treatment, or produce more energy than is needed for treatment.
It would be desirable to develop new systems and methods for wastewater treatment that reduce or eliminate supplemental oxygen need for nitrification of ammonia in wastewater.
It would be desirable to develop new systems and methods for wastewater treatment that reduce or eliminate the release of greenhouse gases through heterotrophic respiration.
It would be desirable to develop new systems and methods for wastewater treatment that increase biogas production per unit volume of wastewater treated.
It would be desirable to develop new algal production systems and methods that reduce or eliminate the need for carbon supplementation for autotrophic algal production.
It would be desirable to develop new algal production systems and methods that reduce or eliminate the need for oxygen supplementation for heterotrophic algal production.
It would be desirable to develop new algal production systems and methods that reduce or eliminate the potential for oxygen toxicity for autotrophic algae.
It would be desirable to develop new algal production systems and methods that reduce or eliminate the need for nutrient supplementation for heterotrophic and autotrophic algal production.